The extent- and cell cycle specificity of apoptotic cell death were studied in human leukemia HL-60 cells treated with 1.25-20 mu M etoposide, 0.125-2.0 mu g/ml dactinomycin, 12.5-200 mu M 3-deazaadenosine (c(3)Ado) and 10-50 Gy gamma-irradiation. Flow cytometry was used to measure the fraction of apoptotic cells and determining their cell cycle position. With all agents the extent of apoptotic cell death showed a clear dose- and time-dependency. Specific apoptosis of S-phase cells was found in cultures treated with 1.25-2.5 mu M etoposide. At 5-20 mu M etoposide, 0.125-2.0 mu g/ml dactinomycin and 50-200 mu M c(3)Ado cells from all cell cycle phases responded by apoptosis. Exposure of the cells to 10 Gy gamma-irradiation resulted in apoptosis of G(2)+M-phase cells. At 50 Gy cells from the S- and G(0)/G(1)-phases also entered apoptosis. Our findings indicate that the cell cycle specificity of an anti-cancer agent initiated apoptotic response depends upon the drug/irradiation dose and exposure time used. This demonstrates that the resistance of subpopulations of cells to apoptosis is relative.

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